Lets take a small element of rod on it with very small size
Its total charge on rod is dQ
so we will have potential at the center of curvature due to small point charge is given as
[tex]dV = \frac{kdQ}{R}[/tex]
here given that
radius (R) = a
now the potential due to complete semicircular arc is given as
[tex]V = \int dV[/tex]
[tex]V = \int \frac{k dQ}{a}[/tex]
[tex]V = \frac{k}{a} \int dQ[/tex]
we know that
[tex]k = \frac{1}{4\pi \epsilon_0}[/tex]
now we have
[tex]V = \frac{Q}{4\pi \epsilon_0 a}[/tex]